The rapid increase in the incidence of gram-positive infections—including those caused by resistant bacteria—has sparked renewed interest in the development of novel classes of antibiotics. A class of compounds which have shown potential as useful antibiotics includes the A-21978C lipopeptides described in, for example, U.S. Pat. Nos. RE 32,333; RE 32,455; RE 32,311; RE 32,310; 4,482,487; 4,537,717; and 5,912,226. Daptomycin, a member of this class, has potent bactericidal activity in vitro and in vivo against clinically relevant gram-positive bacterial that cause serious and life-threatening diseases. These bacterial include resistant pathogens, such as vancomycin-resistant enterococci (VRE), methicillin-resistant Staphylococcus aureus (MRSA), glycopeptide intermediate susceptible Staphylococcus aureus (GISA), coagulase-negative staphylococci (CNS), and penicillin-resistant Streptococcus pneumoniae (PRSP), for which there are few therapeutic alternatives. See, e.g., Tally et al., 1999, Exp. Opin. Invest. Drugs 8: 1223-1238.
Despite the promise that antibacterial agents such as daptomycin offer, the need for novel antibiotics continues. Many pathogens have been repeatedly exposed to commonly-used antibiotics. This exposure has led to the selection of variant antibacterial strains resistant to a broad spectrum of antibiotics. The loss of potency and effectiveness of an antibiotic caused by resistant mechanisms renders the antibiotic ineffective and consequently can lead to life-threatening infections that are virtually untreatable. As new antibiotics come to market pathogens may develop resistance or intermediate resistance to these new drugs, effectively creating a need for a stream of new antibacterial agents to combat these emerging strains. In addition compounds that exhibit bacteriacidal activity would offer advantages over present bacteriastatic compounds. Thus, novel synthetic antibacterial agents would be expected to be useful to treat not only “natural” pathogens, but also intermediate drug resistant and drug resistant pathogens because the pathogen has never been exposed to the novel antibacterial agent. Additionally, new antibacterial agents may exhibit differential effectiveness against different types of pathogens.